split sentences in each paragraph and wrap in span

I'm using this to split articles into individual sentences so I can highlight each sentence on mouseover:

$a_text = str_replace('<p>', '', $a_text);
$a_text = explode('</p>', $a_text); // Separate into paragraphs
foreach($a_text as $p) {
    $p_text = explode('. ', $p); // Only spilt on periods followed by a space to preserve URLs in the article.
    $highlight_text .= '<p>';
    $i = 1;
    foreach($p_text as $text) {
        $highlight_text .= '<span class="highlight" alt="'.$i.'">'.$text.'.</span> '; // Wrap each sentence in the highlight class
        $i++;
    }
    $highlight_text .= '</p>';
}
echo $highlight_text;

My problem is that at the end of every paragraph, there is a double period. It will look something like this:

Sentence one. Sentence two. Sentence three. .

Nothing I do removes that trailing space + period from the end of every paragraph, and I can't figure out how it's getting applied.